In the manual focusing of a digital camera, a photographer searches for a focus position where a subject becomes sharpest on a monitoring image by adjusting the focus position of a lens, thereby performing a focusing operation. An assist function called peaking is developed to improve the accuracy of this focusing (Patent Literature 1).
In the peaking, a marker serving as an auxiliary signal is displayed in a sharp portion of the monitoring image. A photographer is able to adjust focusing easily by searching for a focus position where the largest number of markers is displayed on a subject. Compared to the sharpness of the image, the marker has higher visibility than the sharpness of the image, so the peaking function has an effect of facilitating the focusing operation.
In the peaking, it is determined that the portion where the energy of a high frequency component exceeds a threshold in the input image is in focus, and a marker is rendered to a pixel determined to be in focus.